FIG. 1 illustrates a vehicle powertrain. Heavy lines indicate mechanical power flow whereas thin lines indicate flow of transmission fluid. Engine 10 drives torque converter 12 which, in turn, drives gearbox 14. Gearbox 14 may adjust the speed and torque before transmitting the mechanical power to an output shaft. The gear ratio of gearbox 14 is selected by providing pressurized fluid to hydraulically actuated clutches. Pump 16, driven mechanically by engine 10, draws fluid from sump 18. Valve body 20 routes the pressurized fluid to the torque converter and to the clutches within gearbox 14 that establish the desired gear ratio. The fluid also provides lubrication to gearbox 14 and absorbs heat. The fluid then returns to sump 18.
The transmission operates most efficiently when the fluid is at an optimal temperature. When the fluid is too cold, its viscosity is higher increasing parasitic drag. If the fluid gets too hot, the viscosity is too low resulting in increased leakage around the pump and elsewhere. This increased leakage reduces the pressure available from pump 16 reducing the torque capacity of the clutches within gearbox 14. If the fluid temperature remains high for a sufficient period of time, the friction characteristics of the clutches change and shift quality degrades. The temperature of the fluid is controlled by routing the lubrication fluid through cooler 22 and bypass valve 24. The cooler is a heat exchanger with a fluid loop designed to facilitate heat transfer either directly to ambient air, or to an intermediate medium such as liquid coolant. When the fluid temperature is high, lubrication fluid is routed through cooler 22 before entering the gearbox 14. When the fluid temperature is low, on the other hand, bypass valve 24 routes the fluid directly to gearbox 14 bypassing the cooler and thus permitting the fluid to warm up quicker. Note that, although valve body 20 and bypass valve 24 are illustrated in FIG. 1 as distinct components, some embodiments may integrate bypass valve 24 into the valve body.
Bypass valve 24 receives fluid from a pressure circuit 26 in valve body 20. The pressure in pressure circuit is maintained at a sufficient pressure to force fluid through the bypass valve, cooler, and lubrication circuit 28 at a sufficient flow rate to provide adequate lubrication and heat dissipation. For example, the pressure circuit 26 may be fluidly connected to a torque converter return circuit. When the temperature of the fluid is low, bypass valve 24 fluidly connects pressure circuit 26 to lubrication circuit 28. Hydraulic circuits are fluidly connected when there is a pathway between the circuits that doesn't have any intentional or large flow restrictions, such that the two circuits are at substantially the same pressure. Hydraulic circuits may be separated by pumps or by orifices, of fixed or variable size, that are intended to create pressure differentials between the circuits. Bypass valve 24 may also block flow through the cooler when the fluid temperature is low. Even if the flow is not blocked, the vast majority of the fluid bypasses the cooler due to the cooler's flow resistance. When the fluid is above the optimal temperature, bypass valve isolates pressure circuit 26 from lubrication circuit 28, fluidly connects pressure circuit 26 to cooler supply circuit 30, and fluidly connects cooler return circuit 32 to lubrication circuit 28. This forces the fluid to flow through the cooler.